The Kolbe-Schmitt reaction is a well known commercial route for the synthesis of salicylic acid. Lindsey and Jeskey have described the Kolbe-Schmitt synthesis as well as the present day industrial process in Chemical Review, Vol. 57, pp 584-591 (August, 1957). Other detailed descriptions are found in Thorpe's Dictionary of Applied Chemistry, 4th Edition, Volume 10, pp 660-665 (1950); and in Kirk-Othmer Encyclopedia of Chemical Technology, Second Edition, Volume 17, pp 724-727 (1968); and Hardy's article "Salicylic Acid" in Industrial Eng. Chemistry, Volume 69, pp 55A-56A (1957).
Improvements in the synthesis have been developed, and in the past these efforts have been mainly concerned with increasing the yields of salicylic acid by higher conversions of the phenol fed to reactor. These improvements have led to the commercial processes now in general use.
In these processes the carbon dioxide is introduced at temperatures below about 140.degree. C., preferably between 100.degree.-125.degree. C., and only after the equimolar amount of carbon dioxide has been absorbed at this low temperature, is the temperature gradually riased to above 140.degree.-150.degree. C., typically to 150.degree.-160.degree. C. to effect the rearrangement of the sodium phenylcarbonate into the desired sodium salicylate. Any unreacted phenol is stripped from the reactor in vacuum and recovered for recycling. By the present day commercial processes the amount of phenol, which has to be removed at the end of the cycle, can be kept low, generally around 10-15% of the phenol initially charged to the reactor or autoclave.
The crude sodium salicylate product is dissolved in water and treated with activated carbon to remove color. Acidification with strong mineral acid precipitates the salicylic acid, which is then recovered by, for example, centrifugation. In general, mineral acids such as sulfuric and hydrochloric are employed for this purpose. The salicylic acid thus produced is of good quality generally containing about 99.5% salicylic acid. The main impurity being 4-hydroxybenzoic acid and 4-hydroxy-isophthalic acid. Whereas the established commercial processes give a relative high degree of conversion of the phenol charged and also give an acceptable purity of the end product, they are also characterized by an unrecoverable loss of phenol and a partly recoverable loss of product, which with rising costs of raw materials and energy becomes intolerable.
Although the end product, the commercial salicylic acid, may contain 99.5% salicylic acid and only about 0.5% by-products, the sodium salicylate only constitutes about 90-95% of the sodium salts of organic acids present in the crude carboxylation reaction products. The remainder being salts of 4-hydroxybenzoic acid (4-HBA), 2-hydroxy-isophthalic acid (2-HIPA), and 4-hydroxy-isophthalic acid (4-HIPA).
The known carboxylation conditions will typically produce the various acids in the following amounts:
______________________________________ Mole % ______________________________________ Salicylic Acid 90-95 4-HBA 5-10 2-HIPA 0.1-0.5 4-HIPA 0.5-2 ______________________________________
The by-product acids, especially 4-HBA, are much more soluble in water than salicylic acid itself, and thus they are easily separated from the salicylic acid by precipitating the salicylic acid from a dilute solution at an elevated temperature. This has in the past often led manufacturers to the conclusion that salicylic acid was the sole product of the carboxylation. A typical operation would be to dilute the crude salts to about 1 ton sodium salicylate in 8 to 10 tons of water and precipitate the salicylic acid by adding sulfuric acid at 50.degree.-60.degree. C. This would give a salicylic acid of 99.5% purity in a yield of about 85% of the salicylic acid present. A second impure product could be isolated by cooling the mother liquid to about 25.degree. C., and by recycling this product for purification the total yield could be raised to about 95% of salicylic acid present. But generally an amount of salicylic acid equal to the amount of by-product acids produced will be lost with the mother liquid. In principle the acids could be recovered from the mother liquid by extraction with a solvent followed by evaporation of the solvent. Apart from adding to the capital cost and energy consumption, such additional process steps would only produce a 50:50 mixture of salicylic acid and by-product acids of little, if any, commercial value.
The hydroxy isophthalic acids are not so easily removed during the isolation of the salicylic acid. To produce a high purity salicylic acid the technical product is sublimated leaving the hydroxy isophthalic acids behind in the residue. Many of the problems caused by side reactions as well as the formation of undesirable by-products are discussed by G. A. Korzenovskii, J. Chem. Ind., (USSR), Volume 2, pp 541-2 (1929), which is abstracted in Chemical Abstracts, Volume 24, 838.sup.5.
It follows therefore that it would be desirable to have a process in which the amount of non-salicylic acids formed during the carboxylation step would be significantly reduced.